1. Field of the Invention
The invention relates to an instrumentation module support ring for rotors of rotary machines, and in particular turbo-generator sets, designed to withstand accelerations in excess of 5,000 m/s.sup.2 and comprising two half-rings clamped to the rotor shaft so as to remain attached thereto for speeds up to and in excess of the maximum speed of the rotor.
2. Description of the Prior Art
By the term rotary machine rotor instrumentation is meant apparatus which is fitted to the rotor of an existing machine in order to determine under real operating conditions various operating parameters, in particular mechanical and thermal parameters. This equipment comprises parameter sensors, devices associated with the sensors to generate electrical signals representative of the parameters measured, equipment for displaying, recording and processing these electrical signals, disposed externally of the rotary machine, and coupling means for connecting the devices associated with the sensors to a power supply and to external equipment. These coupling means are generally coreless transformers consisting of coaxial loops associated in pairs, one loop of the pair being fixed and the other rotating with the rotor around its axis.
The sensors are naturally disposed at locations on the rotor at which the parameters to be measured are of particular significance. On the other hand, the associated equipment and the rotating loops are disposed at locations which are appropriately distanced from the rotor shaft and mounted on support rings attached to the shaft at these locations. For convenience and safety when mounting the associated devices on a support ring, the devices are often divided into modules of comparable weight, shape and dimensions.
The present invention was developed in the context of instrumentation for the rotors of high-power turbo-generator sets. As the cost of producing electrical energy, particularly at the base level, is inversely proportional to the unit power of the turbo-generator set (or of the combined plant, grouping the turbo-generator set(s) with the associated steam raising installation, especially when nuclear-powered), the trend is for turbo-generator sets to be increasingly powerful and, as a corollary of this, for the shaft lines to be lengthened for technical reasons and for the reliability of the set to be enhanced for operating reasons, by virtue of the cost involved in shutting down the plant. Note that nuclear-powered plant is designed to be shut down annually, their period of uninterrupted operation being extendable in emergency to 18 months.
The shaft lines of the more recent sets are up to some 50 meters long, with rotating masses of several hundred tonnes, more or less concentrated. The behavior of the shaft lines, especially in response to high-amplitude disturbances on the distribution network, such as nearby faults, the impedance of which as seen by the alternator is very low, is subject to intensive research, not only to specify the torsion fatigue strength of the shafts but also to analyze transient electrical conditions which originate in the alternator as a consequence of such disturbances, by virtue of the mechanical forces on the shafts.
In parallel with the lengthening of the shaft lines, the increase in the power rating results in an increase in the shaft diameter needed to transmit the torque. The shaft diameter typically varies between 500 and 700 mm according to the unit power rating. The levels of centrifugal acceleration at the periphery of these shafts respectively amount to 25,000 and 8,750 m/s.sup.2 or, taking the acceleration due to gravity g as a reference, approximately 2,500 g and 875 g. It need hardly be said that an instrumentation support ring must be designed to safely withstand the maximum centrifugal acceleration involved.
However, it is necessary to distinguish between the degrees of seriousness of the defects which may affect the ring, in order to evaluate the reliability required with regard to such defects. In order of increasing seriousness consideration must be given to: (a) defects concerning only the instrumentation, (b) defects which may compromise the availability of the set, (c) defects which make an emergency shutdown imperative and, finally, (d) defects resulting in serious damage to equipment and injury to personnel.
An emergency shutdown is achieved in approximately 10 minutes by venting to atmosphere the steam feeds to the various parts of the turbine; a normal turbo-generator set shutdown cycle, up to the time at which it is possible to work on the set, routinely takes three days, and to bring the set on line takes about 24 hours, inter alia to avoid distortion of the shaft line due to thermal imbalances.
Defects in category (a) have a nuisance value in that they interrupt the work in progress, but they do not necessitate a premature shutdown of the turbo-generator set. A typical defect of category (a) is a break in the connections from the instrumentation modules to the sensors by virtue of relative rotation between the ring and the shaft, to which it is insufficiently tightly clamped. Defects of category (b) entail a premature shutdown, but under normal conditions; this type of defect relates in practice to the reliability of the support ring, resulting in a "mean time before failure" which is too short, by virtue of mechanical fatigue, for example. Defects of category (c) are such that the seriousness may rapidly escalate to that of category (d). Category (d) defects also entail an emergency shutdown, and steps are taken to alleviate their consequences by means of external protection measures. Distinguishing between defects of categories (c) and (d) is tied to the ability to detect the appearance of the defect in time or not.
Attempts have been made to utilize instrumentation support rings consisting of two metal half-rings fastened together around the shaft and clamped to the shaft by means of a steel band applied when hot. This band was made up of a multiplicity of ring segments disposed on edge to constitute complete rings, bolts passing through all the rings to form a solid block; the gaps between segments in the same binding ring were naturally offset angularly between the binding rings so as to regularly distribute the weak points. The binding, assembled around the shaft beside the support ring, was then heated and slipped over the support ring.
The fitting of a hot steel binding, drawing inspiration from the techniques for constructing vessels subject to high pressures, has the disadvantage in applications to parts subjected to centrifugal accelerations that the mass of the binding is high and increases the forces acting on the building. To give a crude but meaningful approximation, from the stage at which the mass of the binding becomes equal to the mass of the ring which it is retaining in position, the increase in the resistance to centrifugal force obtained by increasing the cross-section of the binding serves more to compensate for the increase in the centrifugal forces due to the increased mass of the binding than to enhancing the binding of the ring.
During instrumentation tests on a 700 MW two-pole turbo-generator set with a shaft having a diameter of approximately 500 mm, loosening of the ring on the shaft was observed, resulting in an offset between the rotor shaft and the instrumentation support ring. Note that in this test the mass of the duralumin ring was of the order of 25 kg whereas that of the binding exceeded 100 kg.
Another proposed solution consists in making a ring of a light and extremely strong material which is sufficiently flexible for clamping of the two half-rings by means of bolts perpendicular to the plane of the mating surfaces (passing through the axis of the shaft) and disposed on the neutral fiber of the ring to achieve attachment of the ring to the shaft in such a way that practically all parts of the contacting surfaces are subject to substantially the same pressure. The material used was a polymerized resin reinforced with glass fibers in a crossed configuration. Tests of a ring of this kind with a mass between 5 and 7 kg on a turbo-generator set proved disappointing. After 20 days in service of the turbo-generator set equipped with this lightweight ring, circumferential cracks were found at the point of junction of the perimeter of the ring and the holes accommodating the clamping bolts. It should be noted that, in spite of these cracks, the ring was still locked tightly onto the shaft; even so, it was obvious that the mean time between failures was insufficient.